1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support, a photoconductive layer formed on the support, and a protective layer formed on the photoconductive layer, which comprises as the main component a polyol curing-type urethane resin.
2. Discussion of Background
Conventionally, a variety of electrophotographic photoconductors are known. For instance, there are known an electrophotographic photoconductor in which a photoconductive layer consisting essentially of selenium or a selenium alloy is formed on an electroconductive support; an electrophotographic photoconductor prepared by dispersing an inorganic photoconductive material, such as zinc oxide or cadmium oxide, in a binder agent and coating the dispersion on an electroconductive support; an electrophotographic photoconductor comprising a photoconductive layer which contains an organic photoconductive material such as a mixture of poly-N-vinylcarbazole and trinitrofluorenone, or an azo pigment; and an electrophotographic photoconductor comprising a photoconductive layer which contains amorphous silicon.
Recently a demand for an electrophotographic photoconductor having high reliability, capable of producing high quality images for a long period of time, is increasing. In the case of an electrophotographic photoconductor with an unprotected and exposed photoconductive layer, the photoconductive layer is gradually damaged by corona charges applied thereto in the course of a charging process. Furthermore, the photoconductive layer physically and chemically deteriorates in a copying process while it is brought into contact with other members of an electrophotographic copying apparatus. The above-mentioned problems are the main factors for shortening the life of the electrophotographic photoconductor.
To solve the above-mentioned problems., methods of providing a protective layer on the surface of an electrophotographic photoconductor are known. More specifically, there are disclosed a method of forming an organic film on the surface of a photoconductive layer of an electrophotographic photoconductor in Japanese Patent Publication 38-15446; a method of providing an inorganic oxide layer on the surface of a photoconductive layer in Japanese Patent Publication 43-14517; a method of successively overlaying an adhesive layer and an insulating layer on a photoconductive layer in Japanese Patent Publication 43-27591; and methods of laminating an amorphous silicon (a-Si) layer, an a-Si:N:H layer or an a-Si:O:H layer on a photoconductive layer by the plasma CVD or the photo CVD in Japanese Laid-Open Patent Applications 57-179859 and 59-58437.
However, when the above-mentioned protective layers have a resistivity of 10.sup.14 .OMEGA..cm or more, which is considered to be too high in electrophotography, the residual potential of the photoconductor increases while in use, and the residual electric charges are gradually accumulated during the repetition of copying operation, which will hinder the practical operation of the photoconductor.
In order to cover up the above-mentioned shortcoming of the protective layer, there is proposed in Japanese Patent Publications 48-38427, 43-16198 and 49-10258, and U.S. Pat. No. 2,901,348 methods of forming a photoconductive protective layer on a photoconductive layer. In addition, there are disclosed a method of adding to a protective layer sensitizers such as dyes and charge transporting agents represented by Lewis acids, as in Japanese Patent Publication 44-834 and Japanese Laid-Open Patent Application 53-133444; and a method of controlling the resistivity of a protective layer by adding thereto finely-divided particles of metals or metallic oxides, as in Japanese Laid-Open Patent Application 53-3338.
According to the above-mentioned methods, however, light applied for image formation is partially absorbed in the protective layer while passing therethrough. As a result, the amount of the light which reaches the photoconductive layer is decreased and accordingly the photosensitivity of the photoconductor is decreased.
To eliminate the above-mentioned disadvantage, there is proposed in Japanese Laid-Open Patent Application 57-30846 a method of providing a protective layer which is made substantially transparent to visible light by dispersing therein finely-divided particles of a metallic oxide having an average particle diameter of 0.3 .mu.m or less, which serve as a resistivity-controlling agent. In a photoconductor provided with the above-mentioned protective layer, the reduction in the photosensitivity can be minimized, the mechanical strength of the protective layer is increased, and the durability thereof is improved.
Especially when a polyol curing-type urethane resin is contained in a protective layer as a binder resin, the wear resistance of the obtained photoconductor can be significantly improved, and the life thereof can also be remarkably prolonged- However, the residual electric charges tend to be accumulated in this photoconductor during the repetition of copying operation, and the residual potential thereof increases, so that toner deposition on the background of images occurs. Specifically, under the conditions at low temperature and low humidity, the above-mentioned problem easily tends to occur, so that this photoconductor is not suitable for use in practice.